What does it mean when my environment has a "dirty" status?

As opposed to “active”?

When you make a change to a project, such as allowing outgoing emails to an environment or allowing indexing by search engines, those changes require that project to be rebuilt and redployed in order to take effect.

The project exists in a state between “Active” and “Inactive” at this time, and will represent itself as “Dirty”/“Building” during this process.

Once the project has been rebuilt and redeployed with those changes, it will once again be shown as “Active”.